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14Статья поступила в редакцию 03.07.13. Ред. рег. № 1710
The article has entered in publishing office 03.07.13 . Ed. reg. No. 1710
УДК (PACS) 621.4/C-601
КОНЦЕНТРАТОРНАЯ УСТАНОВКА С ДВУСТОРОННИМ ОБЛУЧЕНИЕМ СОЛНЕЧНЫХ ЭЛЕМЕНТОВ С ВЕРТИКАЛЬНЫМИ Р-N ПЕРЕХОДАМИ
1 12 В.В. Симакин , А. В. Смирнов , И.И Тюхов
всероссийский электротехнический институт имени В. И. Ленина (ВЭИ) 111250 Москва, ул. Красноказарменная, д. 12 Тел./факс: (495) 361-94-56, e-mail: vsimv@mail.ru 2Московский государственный машиностроительный университет (МАМИ) 105066 Москва, ул. Старая Басманная, д. 21/4
Тел.: (499) 267-10-17, факс: (499) 267-96-12, e-mail: ityukhov@yahoo.com
Заключение совета рецензентов: 08.07.13 Заключение совета экспертов: 13.07.13 Принято к публикации: 18.07.13
В статье представлены способы использования солнечных элементов c вертикальными р-n переходами в солнечных концентраторах. Оригинальной особенностью таких солнечных элементов является наличие двух идентичных чувствительных поверхностей. Показаны эффективные решения, которые были смоделированы и протестированы на созданной установке. Эта установка позволила воспроизвести и протестировать работу всех элементов системы. Статья описывает солнечные элементы c вертикальными р-n переходами, концентратор солнечной энергии, охлаждающую жидкость, солнечную систему слежения и т.д. Созданная система состоит из двух симметричных «крыльев» концентратора, приемно-преобразовательного блока и соответствующего оборудования для контроля электрических, тепловых и других параметров. Преимущества появляются при использовании системы в целом. Концентратор солнечной энергии обеспечивает двустороннее облучение с концентрацией около 20 крат с равномерным распределением излучения по двум принимающим поверхностям солнечных элементов. Приемно-преобразовательный блок состоит из солнечных элементов с вертикальными р-n переходами (в сборе представляет собой линейный приемник), помещенный в прозрачную жидкость с хорошими диэлектрическими свойствами, а также с разъемами, трубопроводами и корпусом с двумя входными кварцевыми окнами. Приемно-преобразовательный блок производит электроэнергию и тепло (PV/T - приемник в соответствии с принятой за рубежом терминологией). Были проведены натурные испытания прототипа PV/T - системы на крыше ВЭИ в Москве.
Ключевые слова: солнечные элементы c вертикальными р-п переходами, солнечный концентратор с двухсторонним облучением, PV/T - система, генерация тепла и электричества.
TWO-SIDED CONCENTRATION SYSTEM FOR VERTICAL P-N JUNCTION SOLAR CELLS
V.V. Simakin1, A.V. Smirnov1, I.I. Tyukhov2
'Department of Renewable Energy All-Russian Electrical Engineering Institute (FGUP VEI), Moscow, Russia vsimv@mail.ru, s_as84@mail.ru 2Moscow State Engineering University UNESCO Chair "Ecologically clean engineering" 21/4 Staraya Basmannaya St., Moscow, 105066, Russia Tel.: (499) 267-10-17, fax: (499) 267-96-12, e-mail: ityukhov@yahoo.com
Referred: 08.07.13 Expertise: 13.07.13 Accepted: 18.07.13
In this paper the ways of using vertical p-n junction solar cell in solar concentrators presented and discussed. The original feature of such solar cells is the identical bifacial sensitive surfaces on both sides of solar cells. Effective solutions, which were simulated and tested on special laboratory installation, are shown. This installation allowed imitating all units of system. Paper describes vertical p-n junction solar cells, concentrator of solar energy, cooling liquid, solar tracking system, etc. The created system consists of symmetrical two-wing concentrator, receiving-conversion unit and proper electrical, heat and control equipment. The benefits appeared when we use them altogether. The solar energy concentrator provides two-side illumination with concentrating ratio about 20x with uniform distribution of radiation on the both receiving surfaces of solar cells. The receiving-conversion unit consists of solar cells with vertical p-n-junctions (assembled in linear receiver suitable to linear focal area) merged
International Scientific Journal for Alternative Energy and Ecology № 11 (133) 2013
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into a transparent liquid with good dielectric properties, and also with connectors, pipelines and box with two entrance quartz windows. The receiving-conversion unit produces both electricity and heat (PV/T - according to accepted terminology). Some field tests were made with prototype of PV/T system on the roof of VEI in Moscow.
Keywords: vertical p-n junction solar cell; two-sided concentrator; solar tracking system; cooling system.
1. Introduction
The purpose of the paper is to describe and discuss some critical engineering difficulties and new opportunities of using vertical p-n junction solar cells. This type of solar cells was invented many years ago. With practically and with reasonable characteristics they were firstly developed in Russia at the end of the sixties of the 20th century [1, 2] (more detailed history see in paper [3]). At that date the planar design was more suitable for non-concentrating light and became the mainstream of PV - the first generation of solar cells still up to now is "working horse" of solar electricity.
Concentrating photovoltaic (CPV) has advanced considerably in the last few decades. At first glance, it seems that the main effort and money must be invested in solar cells with the highest efficiency. Indeed, the great financial resources were invested into the concentrator technology to produce tangible results. For example, concentrator company Amonix in Las Vegas created a production area of 20,000 square meters and has mastered the automated production of concentrator modules on a scale of about 100 MW per year. It was demonstrated in the CPV-7 conference in Las Vegas in April 2011.
At the same time some experts believe that triple-junction solar cells viable in outer space, not on earth.
Also the questions about resource limitations and environmental problems of global world energy rise more and more acute in recent years. The predicted energy demand will reach 28 TW by 2050 and 46 TW by 2100. Deployment of solar PV technology as a source of electricity will have to increase to a scale of tens terawatts in order to become a significant source of energy in the future.
Most of the current commercial and emerging solar cells technologies have natural resource constraints that do not allow them to reach terawatt scale. The silicon vertical p-n junction solar cells technology is out of these restrictions [3].
On the new stage of technological development silicon vertical p-n junction solar cells in concentrators can be more competitive from the points of the cost and availability resources with flat-panel PV and even now close to commercial applications [4, 5].
In comparison to development made by Greenfield Solar Company [5] in this paper we describe how to use the main feature of such solar cell - identical bifacial sensitive surfaces in both side.
Other benefit from using this type of solar cell with concentrator is possibility to create more effective
photovoltaic/thermal (PV/T) system which could provide consumers by both electricity power and hot water for domestic purpose because it already has a cooling system and one needs only heat exchanger and tanks for water.
2. Vertical p-n junction solar cell
The solar cells with vertical p-n-junctions are known as a very good alternative to planar concentrator solar cells. With practically the same efficiency they are more suitable for conversion concentrated radiation [6, 7]. They have a number of advantages: high temperature tolerance, low series resistance, bifacial sensitive accepting surfaces suitable for two-sided illumination, low equilibrium temperature, quite simple technology of manufacturing, etc. Inherent low series resistance gives them opportunities to work under high concentration ration. Previous theoretical investigations showed that the high limiting efficiency of solar cells with vertical p-n-junctions is actually close to traditional planar solar cells [6].
A method of manufacturing solar cells with vertical p-n-junctions using thermal compression bonding with silumin was developed and solar cells were investigated in FGUP VEI and defended by Russian patents. The main goal of this technology is to provide a structure for a photovoltaic cells which gives in a result improved characteristics: improving quality of interconnecting soldering, eliminating of compensative influence of aluminium to n-layers, high temperature tolerance and mechanical firmness. The main stages of quite simple technology for producing solar cells with vertical p-n-junctions (without some steps, for example, such as antireflection coating) are showed in Fig. 1. Firstly, wafers are assembled with p-n-junctions and aluminium or/and silumin inter-layers foils in stack, then, they are moulded together in special order and after that, are cut on plates (Fig. 2).
Рис. 1. Основные этапы изготовления солнечных элементов с вертикальными р-л-переходами Fig. 1. The main steps of fabricating solar cells with vertical p-n-junctions
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Рис. 2. Солнечные элементы с вертикальными р-л-переходами Fig. 2. Solar cells with vertical p-n-junctions
According to our approach, the silumin (alloy of Al and Si) layers prevent a penetration of compensative impurities (Al) into n-region. The produced solar cell structures result in the high quality mechanical and electrical contacts, high fill factor for I-V-characteristic (more than 0.8), and increased tolerance to high temperature under high intensity and high tolerance to penetrating particle radiation (for space applications).
For optimization of technology processes the modern high resolution scanning microscopy methods of investigating vertical p-n-structures were used. Also some new technological improvements were elaborated in addition to base patented technology (described at [7]).The new improvements are at the stage of patenting.
3. Two-sided concentrator
Рис. 3. Концентратор для двустороннего облучения Fig. 3. Two-sided concentrator
Two-sided concentrator was designed in FGUP VEI and showed in the Fig. 3. It has all aforementioned properties. Also it is equipped by planar photovoltaic module in order to decrease loses from blind zone and also increase reliability of solar tracking system in case of absence power supply.
Optimization of concentrator The special program was created in order to optimize solar concentrator shape and size for different concentration rates. This program uses principles of simulating ray tracing and it allows modelling of concentrator's focal spot depending on different manufacturing tolerance and solar tracking system accuracy.
Concentrator of facet type provides more uniform distribution of solar energy along solar cell surface than parabolic one. Selection of concentrator surfaces depends from concentration ratio, size of solar cells and available technology. In our case we choose concentrator of facet type (Table).
Design
The concentrator is the unit of system for intensifying solar ray flow. Concentrators usually consist from mirrors or lenses, which reflect or refract solar rays toward definite area. For creating two-sided lighting of solar cell we should create concentrator which has a few main differences from tradition type of concentrator:
- It has to have two identical wings because in that case each surface will receive similar amount of solar energy and have the same weight and sizes which more important because equality of energy is not critical characteristic and at meantime the centre of mass is very important thing for energy consumption of solar tracking system.
- It has a blind zone under receiver in order to avoid oblique rays fall to solar cells. The blind zone can have different size sand should be optimized for every concentration rate. Usually blind zone has length about 20% of concentrator aperture and it decreases efficiency of concentrator.
- Concentrator should have a rectangle focal spot suitable to rectangle receiver.
Parameters of concentrator Параметры концентратора
Parameter/Information Amount/description
Type Facet
Demanding to manufacturing tolerance ±0,4°
Solar tracking system 2-exis, step-mode
Concentration ratio 2x25
Energy concentration ratio 2x20,5
Dimensions, mm 1000x1000x600
Solar concentrator creates solar beam with width bigger than width of solar cells (about 3-5%). It decreases demands for manufacturing tolerance and accuracy of solar tracking system; and also it increases uniformity of solar energy distribution.
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Solar tracking system Solar tracking system is the critical part of the every concentrator because without it focal area will change position very fast. The TRAXLE Solar Tracker (Poulek Solar Ltd.) was chosen for realization of the concentrator PV/T concept in order to provide tracking concentrator after the Sun (Fig. 7), because the TRAXLE Tracker has reliable and robust construction with easy installation, unattended operation and some other positive features [http://www.solar-trackers.com].
- It can operate in temperatures above 100 degrees;
- It has a good cooling property. Experiments showed that temperature of solar cells in installation with working cooling system can be kept on same level which has 30 degrees difference with environmental temperature.
Finally, the cooling liquid and quartz glass also provide more uniform distribution of solar energy on surface of solar cell.
Modeling results Next results were obtained after modelling and comparison two-sided and one-sided concentrators: if manufacturing tolerance will be equal ±0,4°, solar tracking system has accuracy 0,4° and concentration ratio will be equal 2*25 then efficiency of one-sided concentrator decrease to 65% and two-sided concentrator to 81%. This fact proves that two-sided illumination less sensitive to any errors in manufacturing and working conditions then one-sided.
4. Two-sided receiver
Receiver is a unit where solar cell is placed (Fig. 4). All solar cells are connected in one stack. Receiver has a few unique properties. First of all, it has two sided symmetry. Each side of receiver is closed by special quartz glasses. Second, it is filled with special dielectric cooling liquid.
5. Concentrator PV/T system
Using both electrical and heat energy (PV/T system) is the one of the most promising ways to improve efficiency of these type installations. For this purpose, the model of concentrator was designed (Fig. 5). It has same properties and working parameters as concentrator designed in first stage, but it also has some differences. First, the functional-model of concentrator has jalousie structure facets, not parabolic surface. Second, this design allows changing concentrating ratio during scientific research of concentrator solar cells. The model of concentrator consists of 50 mirrors located in carrying frame. The angle of each mirror may be changed, it necessary for exact adjustment. The carrying frame was fastened on mechanical assembly of two-axis solar tracker.
Рис. 4. PV/T приемник Fig. 4. Assambling of PV/T receiver
Cooling liquid has a direct contact with solar cell so it has a few important properties:
- It is transparent in wide spectrum of solar radiation; therefore it is not dramatically decrease efficiency of system. The experimental work shows that transmittance of liquid is not less than 0.9 for solar radiation with wave length between 400 and 1000 nm;
- It has a good dielectric properties so the solar cells with growing voltage along surface can operate properly and without degradation;
Рис. 5. Эй-изображение PV /T модели концентратора ВЭИ: 1 - основа, 2 - следящая система, Э - промежуточная опора, 4 - несущая рама концентратора, 5 - зеркала, 6 - PV /Т приемник Fig. 5. 3D-image of VEI PV/T model of concentrator: 1 - basis, 2 - mechanical assembly of solar tracker, 3 - intermediate support, 4 - carrying frame of concentrator, 5 - mirrors, 6 - PV/T receiver
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For realization of PV/T systems the experimental concentrator was built and assembled on the VEI roof for testing as it is shown in Fig. 6 [8, 9]. The concentrator allows bifacial illumination and regulating number of illuminated mirrors with help of movable shields located at the tops of concentrator wins and hence allows to measure parameters and characteristics depending on concentration ratio.
I, mA
Рис. 6. Экспериментальный концентратор для PV /Т системы ВЭИ Fig. 6. Experimental concentrator for VEI PV/T system
Рис. 7. ВАХ солнечного элемента без концентратора (а) и с концентратором (b) (Т окр. среды 30 °С, облученность 850 Вт/м2)
Fig. 7. I-V diagrams of solar cell wihout concentrator (a) and with concentrator (b) (ambient Т 30 °С deg., F = 850 W/m2)
6. Measurments and results
Described PV/T system and solar cells with vertical p-n junctions were tested in natural condition. Two I-V diagrams were measured and compared. First diagram for two sided solar cells with concentration ratio 2x (two-sided illumination) and second one for ratio 40x (2x20) are demonstrated at Fig 7.
Maximal power of group of cells is 0.63 W without concentrator and 10.8 W with concentrator for solar radiation with density - 850 W/m2 and ambient temperature 30 deg. It must be taken into account that calculated optical loses in system is about 10% and concentrator does not concentrate diffuse radiation which was equal 10%. Therefore the efficiency of one solar cell increases when concentrator is used even if temperature increases as well.
In this paper research results presented for old samples of solar cells with low efficiency which were used because main purpose of R&D work was testing different solutions for PV/T system, not only for solar cells. Thus, total efficiency of installation is only 13% but with new solar cells achieved efficiency could be about 20% and additional thermal energy for different purposes like preheating can be used.
7. Comparision with planar solar cells
Comparison between vertical p-n junction solar cell (VJSC) and classical planar solar cell presented in Fig. 8. It is obviously from picture that VJSC has few big benefits:
Рис. 8. Сравнение между солнечными элементами с вертикальными р-n переходами и классическими планарными
Fig. 8. Comparison between vertical p-n junction solar cell and classical planar solar cell
International Scientific Journal for Alternative Energy and Ecology № 11 (133) 2013
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- Big rated voltage and low current, thus current loses decrease. It is very important for concentration systems where current increases with solar intensity and loses increased as square of current;
- Better self-cooling properties are explained by better thermal conductivity of Al-layers;
- It can be produced from wastes of power electronics silicon manufacturing (which does not require thin p-n junctions) therefore it can be more cost-saving.
Conclusions
Vertical p-n junction solar cell has two working surfaces so special solutions could be used for more effective practice:
- Two-sided illumination can be obtained by using of solar energy concentrators with special design. This type of concentrator is not as critical to manufacturing tolerance and accuracy of solar tracking system as onesided concentrator with same concentration ratio.
- Additional planar PV module is used to cover and to use blind zone of concentrator for both increasing efficiency and reliability of solar tracking system;
- Special liquid should be used for cooling. It wraps solar cell from both sides so it creates some optical loses but from another side it provides better cooling and more uniform distribution of solar energy on solar cell surface thus it has a compensation effect;
- Described solution could be used in PV/T systems in order to achieve more significant efficiency and also without problems with resource restrictions for large scale applications.
References
1. Landsman A.P., Strebkov D.S. Theoretical and experimental investigation of high voltage photoelectric generator // Fiz. techn. Poluprovodn. (Physics and technology of semiconductors). 1967. Vol. 4. № 10. P. 1922-1928 (There is the translation of journal in English).
2. Strebkov D.S. et al. USSR Author's Certificates № 288159-288163 (1968) (in Russian).
3. Tyukhov I.I. On the resource limitations photovoltaic technologies and prospects for overcoming them // Proc. of the 8th Int. Scientific and Technical Conference (16-17 May 2012, Moscow, GNU VIESH), in 5 parts. Part 4. Renewable Energy. Local energy resources. Ecology. Moscow: GNU VIESH, 2012. P. 4754 (in Russian).
4. Sater B.L., 2003. The development of high intensity silicon vertical multi-junction solar cells. Final report grant DE-FG36-00G010523.
5. http://www.greenfieldsolar.com/- GreenFieldSolar (14 December 2012).
6. Tyukhov I., Vasilev A., Gureev A. Limit efficiency of ideal planar and edge illuminated solar cells Renewable energy for agriculture // Proc. of VIESH (2000). P. 55-79 (in Russian). See also earlier publication in English: I. Tyukhov, A. Vasil'ev Limiting efficiency of the ideal planar and edge-illuminated solar cells. Solar World Congress, Harare, Zimbabwe. Abstracts of the ISES (1995). P. 74.
7. Simakin V., Strebkov D., Tyukhov I. Silicon multi-junctional solar cells with vertical p-n-junctions: evolution, technology, applications, and new opportunities // 14th Int. Solar Conference "Euro Sun 2004".
8. Simakin V., Tyukhov I., Smirnov A. Solar-power plant for generating both electricity and heat // Russian Electrical Engineering, 2010. Vol. 81, No. 3. P. 142-145. Alerton press, Inc., 2010.
9. Original Russian text: Simakin V.V., Tyukhov I.I., Smirnov A.V., 2010, published in Elektrotekhnika, 2010, No. 3, P. 38-42.
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